Arterial oxygen measurements (obtained invasively) are frequently used for measuring the physiological or "effective" shunt fraction resulting from gas exchange units having lower than average ventilation/perfusion ratios. A method employing inert gas insertion and blood concentration measurement (both invasive) of the inert gases has demonstrated the capability of measuring the distribution of low V/Q units and accurately predicting the resulting reduced arterial oxygen concentration. The purpose of this proposal is to demonstrate the feasibility of developing a noninvasive means of both inert gas insertion and arterial blood concentration measurement for the purpose of monitoring the shunt fraction noninvasively. A transcutaneous method, analogous to the transcutaneous method for obtaining arterial oxygen estimates is to be employed. An important difference is that inert gases are undistributed by tissue metabolism, while skin metabolism, coupled with an uncertain skin perfusion, can defect the intent of the oxygen measurements. This gives the inert gas transcutaneous approach a major advantage in that it should not be defeated by physiological uncertainties; however, the major problem with the approach is that sensitivity requirements for the sensing unit exceed the minimum detection limits for the most sensitive gas sensors available. The major objectives of the proposal are 1) to experimentally verify the sensitivity requirements for the method, and 2) construct a working model of a sensing device which meets these sensitivity requirements. A technically favorable feature of the method is that its accuracy depends primarily on gas measurement ratios and the ratios of membrane solubilities of the various gases.